S.J. Bass

Investigation of InGaAs-InP quantum wells by optical spectroscopy

A detailed study of the optical properties of InGaAs-InP single quantum wells (QWS) grown by atmospheric-pressure metal-organic chemical vapour deposition is described. Photoluminescence (PL), photoluminescence excitation (PLE), photoconductivity (PC) and electroreflectance (ER) are employed to study both undoped and modulation-doped quantum wells. The role of extrinsic processes in determining the low-temperature PL spectra is demonstrated from the variation of peak position and linewidth with temperature. The best PL linewidth obtained for a 150 AA well is 5.3 meV, fairly close to the limit imposed by alloy fluctuations in the InGaAs. The role of free carriers in the undoped QWS in determining the energy threshold for optical absorption is demonstrated from a comparison of PLE and PC spectra. Lineshape fitting of the PL spectra is described, and it is deduced that at 160K recombination processes in both doped and undoped QWS proceed with wavevector conservation, although at lower temperatures highly anomalous lineshapes are found in modulation-doped samples. The observation of a threshold in PC spectra under forward bias is interpreted as a transition from the valence-band well to the top of the conduction well. The ratio of the conduction- to valence-band discontinuities is deduced to be approximately 0.4:0.6.

Metal organic vapour phase epitaxy of indium phosphide

Abstract MOCVD layers of InP have been grown over the temperature range 612–725°C in a horizontal atmospheric pressure reactor using trimethylindium (TMI) as the source of indium rather than the more usual triethylindium (TEI). Two variants of epitaxial growth have been used: (1) direct reaction of TMI with PH 3 and (2) an adduct reaction using an in situ prepared adduct of TMI and trimethylphosphine (TMP). Both variants produced layers of good morphology, uniformity and thickness control at growth rates ≈ 3 μm/h. The adduct growth overcomes side reactions although the low volatility limited the maximum growth rate to 4 μm/h. The layers were assessed by capacitance-voltage, 4–300 K Hall and 4–15 K photoluminescence measurements. The undoped layers had carrier concentrations in the range 8×10 14 −×10 16 cm -3 with best room temperature and 77 K mobilities of 4700 and 55,000 cm 2 V -1 s -1 , respectively. A major factor controlling the background carrier concentration is found to be the TMI source. Both electrical and optical assessments show the compensation ratio is low with many layers having θ

InGaAs‐InP multiple quantum wells grown by atmospheric pressure metalorganic chemical vapor deposition

The optical and structural properties of multiple quantum wells of InGaAs‐InP grown by atmospheric pressure metalorganic chemical vapor deposition are reported. Room‐temperature excitons are resolved for well widths from 50 to 200 A. Below 50 K, exciton linewidths, in absorption, of less than 10 meV are obtained. Absorbances for allowed valence to conduction subband transitions are found to be independent of well width, as expected in the two‐dimensional limit. A lower bound for the conduction‐band discontinuity of 235±20 meV is obtained.

High quality epitaxial indium phosphide and indium alloys grown using trimethylindium and phosphine in an atmospheric pressure reactor

Abstract The presence of an unwanted side reaction between phosphine and indium alkyls has presented problems in the epitaxy of indium compound semiconductors. The problem has previously been overcome by the use of low pressures and cracking the phosphine, the use of adduct compounds or the use of carrier gases other than hydrogen. For this work an atmospheric pressure reactor was constructed using hydrogen as the carrier gas. The system is fully computer controlled to give precise control of compositions and a very reproducible growth cycle. Good indium phosphide can be grown without adducts if suitable precautions are taken and optimum growth conditions used. The uniformity of alloys indicates little depletion of the gas stream due to unwanted side reactions. A detailed assessment has been made of the electrical transport properties of the layers by 4–300 K Hall measurements and the mobility analysed using an iterative solution of the Boltzmann equation. The analyses showing the best layers are lightly compensated with Na/Nd ∼ 0.1 and have a total acceptor concentration of few times 1014 cm-3. The background donor is shown to be a shallow hydrogenic centre. The best Hall mobilities obtained to date in 1nP with a background donor concentration of 8 × 1014 cm-3 are 5200 and 74,000 cm2 V-1 s-1 at 294 and 77 K respectively. In lattice matched gallium indium arsenide with a background donor concentration of 1.5 × 1015 cm-3 Hall mobilities of 11,200 and 41,000 cm2 V-1 s-1 at 294 and 77 K have been obtained, showing the electrical quality of MOCVD gallium indium arsenide is comparable to LPE.

Optical properties of InGaAs‐InP single quantum wells grown by atmospheric pressure metalorganic chemical vapor deposition

Low‐temperature photoluminescence and photoconductivity studies of high quality InGaAs‐InP quantum wells grown by metalorganic chemical vapor deposition at atmospheric pressure are reported. The best luminescence linewidth obtained for a 100‐A well is found to be 7.9 meV. The residual line broadening is discussed in terms of interface fluctuations, and is compared directly with structural properties as determined by high resolution lattice imaging transmission electron microscopy.

MOCVD of indium phosphide and indium gallium arsenide using trimethylindium-trimethylamine adducts

It is not yet clear which is the best indium precursor to use for MOCVD of InP and InGaAs. Trimethylindium, triethylindium and adducts of the metal alkyls with group Vb alkyls have been used. For this work indium phosphide and gallium indium arsenide have been grown using trimethylindium-trimethylamine. This adduct has the advantage over the more usual phosphorus alkyl adducts in having a greater volatility. Indium phosphide has been grown under optimum conditions with 298 K and 77 K are 5000 and 46000 cm2 s-1 V-1. Indium gallium arsenide lattice matched to indium phosphide has a carrier concentration of 2 × 1015 cm-3 electron and mobilities at 298 and 77 K of 8000 and 54000 cm2 s-1 V-1 n-type. Control of alloy composition is good. Photoluminescence at 2 K indicates material of good quality and reasonably low compensation.

Photoluminescence study of the density-of-states between Landau levels in the quantum hall effect system

Abstract Very well resolved electron Landau levels (LLs) are observed in the photoluminescence spectra of modulation-doped InGaAsInP quantum wells. An upper limit for the density-of-states (DOS) between LLs of 0.036 D 0 is reported, at a filling factor of ν = 4, where D 0 is the two dimensional DOS at zero magnetic field. This spectroscopic value contrasts strongly with that deduced from measurements sensitive only to the form of the DOS at the Fermi energy. Anomalies in the Landau level transition energies as a function of field are discussed.

Effect of growth temperature on the optical, electrical and crystallographic properties of epitaxial indium gallium arsenide grown by MOCVD in an atmospheric pressure reactor

Indium gallium arsenide was grown epitaxially by metalorganic chemical vapour deposition (MOCVD) on (100) indium phosphide using trimethylindium. At an optimum growth temperature of 680°C, material of very good properties can be grown. The carrier concentration is 1x1015 cm-3 n-type with a mobility 80000 cm2 s-1 V-1 at 7 K. Double crystal X-ray diffraction rocking curve widths are <30 arc seconds using the (400) reflection with Cu radiation and the non-dispersive mode. The peak width in photoluminescence spectra at 2 K is < 2 meV. At lower growth temperatures the photoluminescence deteriorates dramatically. In previous work this has been attributed to enhanced carbon incorporation. In this work X-ray diffraction and transmission electron microscopy (TEM) measurements indicate that the deterioration is structural in origin and possibly due to the onset of longwavelength spinodal decomposition. TEM studies show that the interface between alloy and substrate is essentially defect free for optimally grown near lattice matched layers. Interfaces showing 2D electron gas effects and quantum wells can readily be grown. For wells, pauses in the growth can be used to sharpen interfaces and produce quantum wells of high photoluminescence efficiency.

Transport properties and persistent photoconductivity in InP/In0.53Ga0.47As modulation‐doped heterojunctions

This paper reports a study of the electrical properties of a systematic series of InP/In0.53Ga0.47As modulation‐doped heterostructures grown by metalorganic chemical vapor deposition. Both Hall‐effect and Shubnikov–de Haas measurements are used to obtain consistent values for carrier densities and mobilities. The heterostructures are shown to display a persistent photoconductive effect at low temperatures (<80 K) which results in changes in both the carrier density and the mobility. The variation of mobility with carrier density is analyzed to show that alloy disorder and background charged impurity scattering are the dominant scattering mechanisms. Excitation across the InP band gap is shown to be necessary for the persistent photoconductivity. We propose a mechanism for this effect in which electron hole pairs created by illumination are separated by electric fields built into the heterojunction with the holes subsequently being trapped in the InP substrate.

Free-carrier effects on luminescence linewidths in quantum wells

The effects of free‐carrier broadening on luminescence linewidths in InGaAs‐InP quantum wells (QW’s) are demonstrated using Schottky barrier depletion and magnetic field quantization of the conduction‐band‐energy levels. After removal of free‐carrier broadening, linewidths of 5 meV for 100‐A, and 3.5 meV for 150‐A QW’s grown by metalorganic chemical vapor deposition are obtained. Widths of 3.4 meV for a 110‐A QW grown by molecular beam epitaxy (MBE) on an n+ substrate, and only 2.0 meV at 6.2 T for a similar MBE sample grown on a semi‐insulating substrate are also reported.