WO1988001792A1

WO1988001792A1 - Superlattice for a semiconductor device

Info

Publication number: WO1988001792A1
Application number: PCT/US1987/001894
Authority: WO
Inventors: James N. Eckstein; Dimitry M. Kirillov; Christopher Webb
Original assignee: Varian Associates, Inc.
Priority date: 1986-09-04
Filing date: 1987-08-04
Publication date: 1988-03-10

Abstract

A superlattice used in a high electron mobility transistor achieves desirable three-dimensional transport properties when the thickness of each layer of the superlattice (10) is reduced to less than 20 Angstroems thick, and suitable choice of barrier (20) composition is employed (for AlGaAs less than or equal to 45 % Al).

Description

SUPERLATTICE FOR A SEMICONDUCTOR DEVICE

Field of the Invention

This invention pertains to a superlattice exhibiting three-dimensionail high frequency electron transport properties for a MODFET or similar device.

Background of the Invention

A superlattice is a stack of very thin layers of two different single crystal semiconducting materials. In the case of two crystals having different lattice constants, the structure is referred to as a strained layer superlattice. Superlattices have novel electronic properties apart from bulk crystalline semiconductors. The electronic properties of a superlattice may be tuned by varying the composition, thickness, or periodicity of the layers. In addition, by controlling the strain in a strained layer superlattice, mismatch defects in the crystal can be eliminated. Thus, the superlattice may be used to alter either the electronic or crystallographic nature of the material.

A new type of transistor known as a MODFET or HEMT has been devised to achieve higher switching speeds. It can function as a fast digital switch or as an analog amplifier at frequencies up to 60 gigahertz. The MODFET capitalizes on the fact that electrons move very rapidly in undoped gallium arsenide; signals can therefore travel through it at high speed. The device also accepts and releases considerable amounts of charge rapidly, and it dissipates very little energy in so doing. The device is usually composed of doped aluminum gallium arsenide on undoped gallium arsenide in a field-effect- transistor structure. This device evolved from research on superlattices. Some forms of MODFET contain only two layers of the superlattice. MODFETs have been successfully made using the alloy Al_xGa_l-xAs as the electron donor layer and GaAs as the accumulation and transport layer. In order to accumulate larger numbers of electrons for transport at the heterointerface, the donor layer bandgap is chosen to be as large as pract ically possible. Deep levels known as DX centers appear in copious quantities when x is increased from .22 to .3, giving increased carrier freezeout at cryogenic temperatures and adversely affecting doping efficiency and device noise properties. In order to eliminate DX centers, superlattices consisting of layers of Al_xGa_l-xAs and GaAs with effective bandgaps equivalent to Al_yGa_l-yAs, y<x, as measured by photoluminescence have been fabricated in such a way as to physically separate the Al atoms from the Si donor atoms. MODFETs or HEMTs with such a material for the donor layer have been fabricated.

The superlattices of Al_yGa_l-yAs and GaAs that have been used to replace Al_xGa_l-xAs in MODFET or HEMT structures have been made with a range of layer thicknesses to obtain effective bandgaps in the mix of the alloy x about 0.3. These have not exhibited sufficiently good three-dimensional transport in order to provide for the capability of high frequency channel charge and current modulation.

Objects of the Invention

An object of the invention is to provide semiconducting material suitable for electron donor layers in HEMTs or MODFETs that has a reduced density of deep level traps when compared with Al_xGa_l-xAs, but retains good three-dimensional electron transport properties, and to use this material for donor layers in HEMT or MODFET devices. Summary of the Invention

These objects of the invention and other objects, features and advantages to become apparent as the specification progresses are accomplished by the invention according to which, briefly stated, by keeping the high bandgap layers of a composition that is direct bandgap in bulk material and choosing its thickness small enough, i.e., less than 20 Angstroms, the bulk transport properties of the superlattice itself appear three-dimensional, and this effect allows for high frequency modulation of the channel change and current.

These and further constructional and operational characteristics of the invention will be more evident from the detailed description given hereinafter with reference to the figures of the accompanying drawings which illustrate one preferred embodiment and alternatives by way of non-limiting examples.

Brief Description of the Drawings

FIGS, 1a and 1b show Raman spectra of thick layers of n-type GaAs in FIG. 1a and Ga_0.78Al_0.22As in FIG. 1b. L₁, L₂ and L₃ are coupled plasmon-phonon modes. LO are longitudinal optical phonon lines which appear due to scattering in the surface depletion layer. SO are second order phonon lines. Instrumental linewidth is shown.

FIGS. 2a and 2b show Raman spectra of n-type doped GaAsGa_0.78Al_0.22As multiple quantum well structures with different layer thicknesses.

Layer thic kness is shown . FIG . 2a shows d isappearance of the plasmon peak when two-d imens ional behavio r dom inates . FIG . 2b shows reappearance of the plasmon pe ak when the electron gas becomes three-d imens ional . Note the strong damping of the plasmon mode when predominantly two-dimensional type electron gas behavior is established, d = 100, 50 Å, and reappearance of the plasmon mode for thinner layers, d ≤ 20 Å, when three-dimensional electron gas is formed and plasmons can propagate across the layers.

FIGS. 3a, 3b and 3c show Raman spectra of n-type GaAs-Ga_0.7Al_0.3As superlattices with different types of doping. In FIG. 3a only Ga_0.7Al_0.3As layers are doped, n is about 2.8 x 10¹⁸cm^-3. in FIG. 3b only GaAs layers are doped, n is about 5 x 10¹⁸cm^-3.

In FIG. 3c both layers are doped, n is about 4.5 x 10¹⁸cm^-3. carrier concentration was evaluated from the frequency of the L₃ plasmon mode. FIG. 4 shows Raman spectra of n-type GaAs-Ga_l-xAl_xAs superlattices with different Al content in the barrier layer. The Al content is shown. Note the increase in linewidth and decrease in frequency of the L₃ plasmon mode which corresponds to the decrease in electron mobility and electron concentration. Electronic contribution to the spectra disappears entirely for x = 0.7 and x = 1.

FIG. 5 shows scattering by folded acoustic modes of 10Å Ga_l-xAl_xAs superlattices with different Al content. Frequencies of the modes and Al content are shown. Note the increase of mode intensity with increase of Al content.

FIG. 6 shows a section through an FET using the superlattice according to the invention.

Glossary

The following is a glossary of terms, elements, and structural members as referenced and employed in the present invention. 10 - superlattice 12 - substrate 14 - gate 16 - source 18 - drain

20, 22, 24 - layers of superlattice 26 - accumulation region

Detailed Description of the Preferred Embodiments As in homogeneous semiconducting materials, collective excitations of the free electron gas, plasmons, can exist in doped multiple quantum well structures and superlattices. In uncoupled or weakly coupled quantum wells, only quasi-two-dimensional plasmons can propagate along the layers of a heterostructure [See S. Das Sarma and J.J. Quinn, Phys., Rev. B25, 7603 (1982), and D. Olego, A. Pinczuk, A.C. Gossard, and W. Wiegmann, Phys. Rev. B26, 7867 (1982)]. Plasmons are strongly damped for propagation along the superlattice axis direction because electrons cannot move across the potential barriers and are localized in quantum wells. When the thickness of layers becomes small, electrons may tunnel through the potential barriers, and broad energy bands a're formed instead of localized quantum states for electrons moving along the superlattice axis [See R. Dingle, Festkorper Probleme XV, Advances in Solid State Physics, ed. H.J. Queisser (Pergamon Press, NY, 1985), p. 21]. In this case, the transition from the quasi-two-dimensional electron gas to three- dimensional electron gas takes place, and plasmons may propagate in any direction in a superlattice.

We applied Raman scattering to study the transition between quasi-two-dimensional and three-dimensional behavior of plasmons. When the backscattering configuration is used with the incident and scattered light propagating along the superlattice axis, the momentum conservation law requires that plasmons participating in scattering also propagate along the same axis. Studying the series of superlattices with decreasing thickness of layers, we were able to observe the appearance of propagating plasmon modes in the spectra and the establishment of three-dimensional behavior. All of the materials used were prepared by MBE using a Varian Gen-II system equipped with non-indium substrate handling and the ability to accept up to 3"-diameter wafers. For substrate temperature measurement, a Minolta Land Cyclops 52C optical pyrometer was used, which has a silicon detector with quoted sensitivity from 0.8 to 1.1 micro-m. This sensitivity range is near optimum, being determined by the requirements that the wafer be both opaque and emitting strongly at the wavelength and temperature ranges in question [See D.E. Mars et al, J. Vac. Sci. Technol.

B4, 571, 1986]. The flux ratio (As₄/GA+AI) was maintained at about 1.3. In all cases, growths were sequential on semi-insulating substrates: undoped GaAs (about 0.3 micro-m) , undoped AlGaAs (about 0.3 micro-m) , and the superlattice (0.4 micro-m) doped as described below. For studying the effect of layer thickness, it was convenient to dope all layers uniformly; GaAs-Ga_l-xAl_xAs superlattices with x = 0.22 were studied so that trapping effects would not be too severe. However, for studying the effect of different x values in the barrier, the layer thicknesses were fixed (both GaAs and AlGaAs) at 10Å and planar doping utilized, i.e., the growthwas stopped halfway through the GaAs layer and Si deposited in an amount so as to keep the overall doping level comparable with the uniformly doped layers. [See S. Sasa et al, Japan J. Appl. Phys, 24, L602, 1985.] For the 10Å layers at x up to 0.3, it was verified that the Raman spectra were essentially independent of the doping scheme. The reason for using planar doping in the second case was to avoid unwanted trapping effects due to doping of Ga_l-xAl_xAs with high Al content.

The uniformly doped layers were grown at a substrate temperature of 540°C, while the planar doped layers were grown at about 450°C. This latter temperature was not measured by the pyrometer, being well out of its range, but represents an estimate scaled from non-contacting thermocouple readings. The use of unconventionally low growth temperatures has been discussed by Ogawa, et "al. [See M. Ogawa et al, Japan J. Appl. Phys., 24, L572, 1985.] We found perfect mirror surface morphology as assessed by phase contrast microscopy. The low temperature was used for the planar doped layers because of the surface segregation effects exhibited by Si in AlGaAs [See S. Sasa et al, Japan J. Appl. Phys., 23, L573, 1984; K. Inoue et al, Appl. Phys. Lett. 46, 973, 1985], and also probably, though perhaps to a lesser extent, in GaAs. For the purpose of this work, it was desirable to be as certain as possible that the intended structure was realized, although it is not clear at this point whether the crystalline quality achieved at such low temperatures is adequate for device applications.

The Raman spectra were taken in the backscattering configuration. Incident and scattered light propagated along the [100] direction in the superlattice, which was the growth axis, with the incident light polarized along the [110] direction. Scattering by plasmons was observed when the scattered light was analyzed along the [110] direction and not observed when it was analyzed in the [1T0] direction. This type of selection rule corresponds to the B2 irreducible representation of the tetragonal D_2d point group of the superlattice, which is expected for longitudinal electrically active waves such as plasmons. An argon ion laser with radiation at 5145 A was used as a light source. The power was approximately 750mW, and the beam was focused by a cylindrical lens to avoid heating or damage of the superlattice. Scattered light was analyzed by a Spex double grating spectrometer with a cooled GaAs photomultiplier. When the low frequency spectral region including folded acoustic phonons of superlattices was studied, samples were put under a glass bell jar filled with helium gas in order to eliminate scattering by rotational excitations of O₂ and N₂ molecules in the air. The penetration depth for the 5145 Å line of an argon laser was in the range of 1000 Å for the materials studied. Due to the backscattering configuration, the collective excitations participating in scattering propagated along the superlattice axis, i.e., across the layers of the superlattice. As follows from momentum conservation, the wave number of plasmons participating in scattering was about 1.6 x 10⁵cm^-1, which corresponds to a wavelength approximately 600 Å.

The spectra of Si-doped layers of GaAs and Ga_0.78Al_0.22As grown by MBE are shown in FIG. 1. The layers have a thickness of approximately 5000 Å, and therefore the spectra correspond to bulk material. The L₁ and L₂ modes in FIG. 1(a) are coupled plasmonphonon modes of GaAs. [See G. Abstrerter, et al, Appl. Phys. 16, 345, 1978.] The free carrier concentration may be determined from the frequency of the L₂ mode, which is equal to 6.1 x 10¹⁸cm^-3. The L₁ , L ₂ and L₃ modes in FIG. 1(b) correspond to coupled plasmon-phonon modes of Ga_l-xAl_xAs alloy. In this case, there are three coupled modes, since there are two longitudinal phonon modes in such alloys in addition to the free electron concentration n is about 3.2 x 10¹⁸cm^-3. [See D. Kirillov, et al, J. Appl. Phys. 59, 231, 1986.] Both layers were grown under identical conditions with the same Si flux. This demonstrates the well known fact that the activation of Si is noticeably lower in GaAlAs than in GaAs.

The superlattices studied consisted of Si-doped GaAs and Ga_l-xAl_xAs layers similar to the bulk material whose spectra is shown in FIG. 1(a,b). While plasmons are also expected to exist in superlattices due to the difference in structures between homogeneous materials and superlattices, the properties of such plasmons are different. [See S. Das Sarme et al, Phys. Rev. B25, 7603, 1982.] When the well thickness is thin enough for the formation of bound electron states, but the barrier thick enough to prevent tunneling, electrons cannot move freely along the superlattice axis, but can move in the plane parallel to the layers. In this case, only two-dimensional plasmons exist. When the layer thickness is made sufficiently small for electrons to tunnel through potential barriers, electrical transport through barriers is established and plasmons are expected to propagate across the layers of a superlattice. Therefore, the three-dimensional behavior of plasmons is restored.

The Raman spectra of doped layer structures with different layer thicknesses are shown in FIG. 2. In the case of thick layers, D = 5000 Å, plasmons are the same as in a bulk material. When the layer thickness is comparable to the wavelength of plasmons, D = 500, 300 Å, the L₂ mode of GaAs, which is the top layer of the structure, dominates the spectrum and starts to be damped due to electron scattering at the layer boundaries. When the layer thickness is equal to 100, 50 Å, three-dimensional plasmons cannot propagate within the layers nor across the layers, and the two-dimensional behavior of the electron gas is established. The high energy plasmon modes L₂ and L₃ are completely damped. The low energy coupled plasmon-phonon modes also show marked changes in this range of layer thicknesses.

When the layer thickness if further reduced to 20 Å, high energy plasmon modes start to propagate again, and they are fully recovered, at D = 10 Å.

There is no further noticeable change in the plasmon spectra when the layer thickness is further reduced to 5 Å This behavior is in good agreement with the Kronig-Penney model for electrons in an infinite array of potential wells of equal thickness. [See R.A. Smith, Wave Mechanics of Crystalline Solids, Ch. 4, Chapman and Hall, London, 1961.] If we use the value of the potential steps in the conduction band of 0.20 eV for the Ga_0.78Al_0.22As barrier and the electron mass equal to that of GaAs, 0.066 m_e, the Kronig-Penney model shows that the formation of minibands and establishment of the electron transport across the layers starts to grow dramatically in the range of the layer thickness of about 25 A. We studied the influence of different methods of doping of the superlattices. Raman spectra of differently doped GaAs-Ga₀.7Al₀.₃As superlattices with 10 Å layers are shown in FIG. 3. FIG. 3a corresponds to doping of Ga_0.7Al_0.3As layers; GaAs layers were doped in FIG. 3b, and both layers were doped in FIG. 3c. As is indicated by the spectra, a three-dimensional electron gas is produced in all three cases, with electrons relaxing to minibands formed by GaAs quantum wells. The act ivation of dopants is noticeably lower when only Ga₀.7Al₀.₃As is doped. This is similar to the case of bulk materials, FIG. 1(a,b).

The influence of the barrier layer composition on the electron gas behavior in 10 Å is demonstrated in FIG. 4. In this case, planar doping of only GaAs layers was used. When the Al content in the Ga_l-xAl_xAs layer is increased, the carrier concentration decreases and damping of plasmons increases, which corresponds to a decrease of mobility. The increase in damping becomes especially apparent at 40% Al and continues through 50% Al. There are no detectable plasmon modes in the case of 70% Al and 100% Al. Although the increase of the barrier height may cause some decrease in the mobility for electron transport across the barrier, we observe much stronger effects. The carrier concentration drops significantly together with a decrease in mobility, as shown by a strong growth in the spectra of unscreened L0₁ phonons of the superlattice for layers with 70% and 100% Al. This anomalous behavior may be due to the presence of electron traps in the layers with high Al content. It is interesting to note that the disappearance of plasmons occurs in the region of barrier composition near the direct/indirect transition.

The Al content in Ga_l-xAl_xAs layers also influenced the intensity of folded acoustic modes. The low frequency parts of the spectrum of

GaAs-Ga_l-xAl_xAs 10 Å layer superlattices are shown in FIG. 5. The behavior was similar for doped superlattices. The frequencies of folded acoustic modes are shown and correspond to the superlattice layer thickness in the range of 10 Å. [See C. Colvard et al, Phys. Rev. B31, 2080, 1985.] The differences in frequencies for different superlattices reflect the precision of the growth. The frequency fluctuations are within ± 5 cm^-1 and correspond in this thickness range to the effective thickness fluctuations less than one atomic layer. The decrease of intensity of the folded acoustic modes shows that the matrix elements for this type of scattering depend noticeably on the height of the potential barrier between superlattice layers.

From our results, it follows that superlattices with three-dimensional electron gas can be grown by MBE. This was proven by the observation of Raman scattering by plasmons propagating in the direction normal to the layers of GaAs-Ga_l-xAl_xAs superlattices. Three-dimensional plasmons appear in the spectra for layer thicknesses below 20 Å when tunneling of electrons across the barriers becomes significant. Damping of plasmons occurs strongly when Al content in the barrier laye rs exceeds 40% , and free electron effects in the spectra disappear in the case of barriers with high content of Al. Materials consisting of superlattices with three-dimensional behavior of electron gas, similar to those studied in the present work, may find applications in a number of electronic devices. These structures may be especially important in the cases when high quality materials may exist only in the form of strained superlattices, such as GalnAs alloys with high In content.

Referring now to the drawings wherein reference numerals are used to designate parts throughout the various figures thereof, there is shown in FIG. 6 an FET incorporating a superlattice 10 according to the invention. A substrate 12 of GaAs supports the superlattice. A gate 14 is formed between a source 16 and a drain 18. The superlattice 10 can be formed, for example, a pair of barrier layers 20 of Al_zGa_l-yAs undoped, layers 22 of Al_zGa_l-zAs undoped, and layers 24 of Al_zGa_l-zAs doped. Only one cycle consisting of two undoped layers 22 and one doped layer 24 and a pair of barrier layers 20 are shown, but many such cycles are used. The parameter z is less than the parameter y; typically y is less than or equal to 0.45 and z is less than or equal to 0.2. The thickness of each pair of barrier layers 20 is less than 20 Angstroms. The accumulation region 26 is formed in the upper region of the substrate. Various semiconductor materials may be used to form the superlattice according to the invention. In general, however, it comprises a first plurality of relatively wider bandgap barrier material, each pair of barrier layers being less than 20 Angstroms thick, and separating these layers with a second plurality of narrower bandgap semiconductor layers, which are interleaved with and contiguous with the first plurality.

The invention is not limited to the preferred embodiment and alternatives heretofore described, to which variations and improvements may be made including mechanically and electrically equivalent modifications to component parts, without departing from the scope of protection of the present patent and true spirit of the invention, the characteristics of which are summarized in the following claims.

Claims

ClaimsWhat is Claimed is:

1. A structure within a semiconductor device, comprising: a first plurality of spaced apart, narrow bandgap semiconductor layers; and a second plurality of wide bandgap layers interleaved with and contiguous with said first plurality; said layers of said second pluralities being each less than 20 Angstroms thick.

2. A structure as in claim 1 wherein each layer of said first plurality of spaced apart, narrow band semiconductor layers comprises multiple sublayers of alternating doped and undoped semiconductor materials.

3. A structure as in claim 2 wherein said first plurality of spaced apart narrow bandgap semiconductor materials comprises A_zGa_l-zAs and said second plurality of wide bandgap layers comprises Al_yGa_l-yAs where the parameter z is less than the parameter y.

4. The structure of claim 3 wherein z is less than or equal to 0.45.

5. The structure of claim 4 wherein y is less than or equal to 0.20.