A. Majerfeld

Very high carbon incorporation in metalorganic vapor phase epitaxy of heavily doped p‐type GaAs

Very high C incorporation (≳1020 cm−3) in GaAs was achieved by atmospheric pressure metalorganic vapor phase epitaxy (AP‐MOVPE) using CCl4 as a dopant gas. Hole densities up to p=1.2×1020 cm−3 (at least three times higher than previously reported by MOVPE) were obtained at a growth temperature of 600 °C and a V/III ratio of 2.8. The highest atomic C concentration was 1.5×1020 cm−3. The hole mobilities were ∼50% larger than previously reported. CCl4 was found to suppress the formation of gallium droplets and whisker growth which normally occur under low‐temperature, low V/III ratio growth conditions, allowing the growth of thin (<1 μm) heavily doped layers with mirror‐like surface morphologies. Layers with p∼1×1020 cm−3 showed a lattice contraction with Δa/a=−9.3×10−4. Photoluminescence studies indicate a significant band‐gap shrinkage at high doping levels.

Heavily doped p-GaAs grown by low-pressure organometallic vapor phase epitaxy using liquid CCl4

A hole concentration greater than 1020 cm−3 in GaAs has been achieved using a liquid CCl4 source for carbon in a low‐pressure organometallic vapor phase epitaxy system. The resistivity and hole mobility measured at 300 K for a heavily carbon‐doped (1.2×1020 cm−3) Hall sample made from a thin (180 nm) epitaxial layer were 8.0×10−4 Ω cm and 65 cm2/V s, respectively. Carbon‐doped samples with excellent surface morphology were achieved using a V/III ratio of 22, and growth pressure and temperature of 80 Torr and 600 °C, respectively. A novel photoluminescence technique, based on band‐gap shrinkage of heavily doped p+‐GaAs, has been shown to be useful for nondestructive measurement of the hole concentration in submicrometer layers.

Heavily doped GaAs:Se. I. Photoluminescence determination of the electron effective mass

A systematic study of the photoluminescence (PL) of Se‐doped n‐type GaAs grown by metalorganic chemical vapor deposition is reported. A new method is presented to determine the electron effective mass of n+‐direct‐gap semiconductors from the PL spectrum. GaAs samples with electron densities from 1015 to 8×1018 cm−3 were investigated over the temperature range of 13 to 353 K. The PL spectra of n+‐GaAs are analyzed using a physical model which for the first time explains in a consistent manner both the energy of the peak and the full width at half‐maximum, and accounts for the electron density. An accurate fit of the PL spectra is obtained by invoking band‐to‐band transitions without k selection. The electron exchange and correlation interactions account for all the observed band shrinkage, which reaches 48 meV for n=8.0×1018 cm−3. No significant density of band‐tail states is observed. The Fermi energy is obtained directly from the PL fitting and is used with the measured Hall electron density n to determi...

Determination of band gap narrowing and hole density for heavily C‐doped GaAs by photoluminescence spectroscopy

The energy band gap narrowing effect in heavily C‐doped GaAs was investigated using photoluminescence spectroscopy. The band gap was determined over the hole density range 1017–4×1020 cm−3 at 10 and 300 K. The band gap data at low temperatures confirm the available theoretical calculations up to 1020 cm−3. An unexpected temperature dependence of the observed band gap at high doping levels is discussed on the basis of carrier‐phonon interactions. We present an analysis of the band gap narrowing effect that can be used for nondestructive measurement of hole densities in the range 1017–4×1020 cm−3.

Strain relaxation and compensation due to annealing in heavily carbon‐doped GaAs

Heavily C‐doped GaAs grown by atmospheric pressure metalorganic vapor phase epitaxy using CCl4 as the C‐dopant source has been annealed to study the stability of C acceptors at very high doping levels (p=1018–1020 cm−3). In layers with initial hole densities p≳6×1019 cm−3, 5 min anneals at temperatures ranging from 700 to 850 °C under arsine overpressure caused a significant reduction in the hole density, lattice contraction and photoluminescence intensity, and a smaller reduction in the mobility. For lower doped material, annealing has little effect on the as‐grown properties. These changes in the material properties indicate that a compensating recombination center is formed during annealing. Possible compensation mechanisms which explain partially the annealing effects in very heavily C‐doped GaAs are analyzed. The results of this study show that there is an upper limit on the hole concentration of p≳6×1019 cm−3 in annealed GaAs:C.

Determination of donor and acceptor densities in high‐purity GaAs from photoluminescence analysis

We report a new analysis technique to determine the acceptor density NA and the donor density ND in high‐purity GaAs from 10 K photoluminescence (PL) measurements. For both n‐type and p‐type samples, we find that NA/ND is related to the excitonic intensity ratio rx=I(A0,X)/I(D0,X) by NA/ND=0.89rx−0.06. In addition, NA can be determined from NA=1014IA, where IA is the emission intensity of the donor‐acceptor pair transition normalized to the intensity of the unresolved exciton peak. Therefore, for n‐type and p‐type material with an impurity density 1013–1016 cm−3, NA and ND can be determined solely from a 10 K PL measurement. The advantage of this technique lies in its nondestructive nature and its applicability to situations where Hall measurements are not possible or suitable.

Heavily doped GaAs:Se. II. Electron mobility

A study of the mobility μ of Se‐doped n+‐GaAs grown by metalorganic chemical vapor deposition is presented. A significant decrease in μ is observed for n>1×1018 cm−3, which is a general characteristic of n+‐GaAs. Previous explanations that the low values of μ are the result of autocompensation by the dopant are unsatisfactory in view of the universality of the decline in μ. A new formula is derived for the ionized impurity mobility μI for degenerately doped material which accurately predicts the experimental μ using no compensation and no adjustable parameters. The formula takes into account the increase of the effective mass m* due to nonparabolicity of the conduction band and due to distortion of the band by the donor atoms. For degenerate material, μI is inversely proportional to the square of m* at the Fermi energy EF. For uncompensated GaAs with n=1×1019 cm−3, m* at EF is 2.4 times m* for pure GaAs, and μ is only 1000 cm2/V s. Previous theories, which use the smaller optical effective mass m*opt in p...

ELECTRONIC AND INTERSUBBAND OPTICAL PROPERTIES OF P-TYPE GAAS/ALGAAS SUPERLATTICES FOR INFRARED PHOTODETECTORS

Existing theories of electronic properties and optical transitions in quantum‐well structures are extended to p‐type superlattices including the two heavy‐ and light‐hole valence bands. These theories are then used to elucidate the normal incidence optical‐absorption mechanisms including the Hartree and exchange‐correlation many‐body interactions on the basis of the one‐particle local density approximation. The effects of doping density and doping configuration on the electronic structure and the intersubband optical properties of heavily doped p‐type GaAs/AlGaAs superlattices are investigated for use in infrared photodetectors. It is shown that these many‐body interactions cause significant changes to the subband energy structure and the optical‐absorption coefficient, and that the doping level and doping configuration have an important effect on the properties of these superlattices. Peak absorption coefficients of 6000–10 000 cm−1 for normal light incidence at photon wavelengths of 8–10 μm are predicte...

Substrate‐induced peak in the photoluminescence of heavily doped epitaxial GaAs

A peak at 1.38 eV is observed in the 300‐K photoluminescence of n+ and p+ epitaxial GaAs. This peak appears as a shoulder of the principal band‐to‐band emission. The shoulder is an artifact produced by the substrate and should not be interpreted as an additional optical transition. It is demonstrated that the subband‐gap luminescence, which arises from the band‐gap reduction caused by the heavy doping, travels through the transparent semi‐insulating substrate, reflects off the back surface, and is emitted from the epilayer. The shoulder intensity is enhanced by the scattering of light off the back surface. Thick substrates with polished back surfaces are optimum for reducing the shoulder peak.

Two‐band analysis of hole mobility and Hall factor for heavily carbon‐doped p‐type GaAs

We solve a pair of Boltzmann transport equations based on an interacting two‐isotropic‐band model in a general way first to get transport parameters corresponding to the relaxation time. We present a simple method to calculate effective relaxation times, separately for each band, which compensate for the inherent deficiencies in using the relaxation time concept for polar optical–phonon scattering. Formulas for calculating momentum relaxation times in the two‐band model are presented for all the major scattering mechanisms of p‐type GaAs for simple, practical mobility calculations. In the newly proposed theoretical framework, first‐principles calculations for the Hall mobility and Hall factor of p‐type GaAs at room temperature are carried out with no adjustable parameters in order to obtain direct comparisons between the theory and recently available experimental results. In the calculations, the light‐hole‐band nonparabolicity is taken into account on the average by the use of energy‐dependent effective ...