N. Amir

Metalorganic chemical vapor deposition CdTe passivation of HgCdTe

CdTe epilayers are grown by metalorganic chemical vapor deposition (MOCVD) on bulk HgCdTe crystals with x ~ 0.22 grown by the traveling heater method (THM). The THM HgCdTe substrates are (111) oriented and the CdTe is grown on the Te face. The metalorganic sources are DMCd and DETe, and the growth is performed at subatmospheric pressure. Ultraviolet (UV) photon-assisted hydrogen radicals pretreatment plays a dominant role in the electrical properties of the resulting heterostructures. The requirements of a good passivation for HgCdTe photodiodes vis-a-vis the passivation features of CdTe/HgCdTe heterostructures are discussed. The effect of valence band offset and interface charges on the band diagrams of p-isotype CdTe/HgCdTe heterostructures, for typical doping levels of the bulk HgCdTe substrates and the MOCVD grown CdTe, is presented. Electrical properties of the CdTe/HgCdTe passivation are determined by capacitance-voltage and current-voltage characteristics of metal-insulator-semiconductor test devices, where the MOCVD CdTe is the insulator. It is found that the HgCdTe surface is strongly inverted and the interface charge density is of the order of 1012cm2 when the CdTe epilayer is grown without the UV pretreatment. With the in-situ UV photon-assisted hydrogen radicals pretreatment, the HgCdTe surface is accumulated and the interface charge density is -4. 1011 cm-2.

Determination of the interface charge between an epilayer and a substrate using capacitance‐voltage measurements

A method is presented for the measurement of the interface charge between an epitaxial layer and a substrate. Capacitance‐voltage characteristics of Schottky contacts formed on the epilayer are measured and analyzed. The interface charge is determined by the modified built‐in potential and capacitance. Experimental results are reported for p‐type CdTe epilayers grown by metalorganic chemical vapor deposition on a p‐type CdTe substrate using indium contacts.

Characterization of CdTe substrates and MOCVD Cd1−xZnxTe epilayers by Raman, photoluminescence and X-ray diffraction techniques

Abstract CdTe substrates and the quality of the Cd 1− x Zn x Te ( x ⩽0.1) epilayers grown by metalorganic chemical vapor deposition (MOCVD) on CdTe substrates, are characterized by Raman scattering and photoluminescence (PL) as well as by X-ray double-crystal rocking curve (DCRC). At a low temperature the intensity of LO phonon is enhanced wherever there is a structural defect. The defect-induced enhancement is due to a large momentum transfer which enhances the intraband Frolich interaction. In addition, the bound exciton peak intensity measured by PL decreases wherever the LO phonon scattering efficiency increases confirming that the defect is the origin of the above Raman enhancement. The quantitative measure of the structural perfection is related to the ratio between the defect band and excitonic peaks in the PL spectra, and correlates with the X-ray full-width at half-maximum (FWHM) of the layer peak. It is shown that in addition to these parameters, the FWHM of the PL defect band is a useful parameter to determine the quality of the epilayer, and a good correlation is obtained between the different parameters. The effect of growth parameters such as zinc partial pressure in the reactor during growth and the reactor design are studied. The results indicate that crystalline imperfection is caused by lattice mismatch between the CdTe substrate and the CdZnTe epilayer and by the nonuniformity of the zinc composition throughout the layers. The quality of the layers is independent of the reactor volume.

The interface of metalorganic chemical vapor deposition-CdTe/HgCdTe

The metalorganic chemical vapor deposition (MOCVD) growth of CdTe on bulk n-type HgCdTe is reported and the resulting interfaces are investigated. Metalinsulator-semiconductor test structures are processed and their electrical properties are measured by capacitance-voltage and current-voltage characteristics. The MOCVD CdTe which was developed in this study, exhibits excellent dielectric, insulating, and mechano-chemical properties as well as interface properties, as exhibited by MIS devices where the MOCVD CdTe is the single insulator. Interfaces characterized by slight accumulation and a small or negligible hysteresis, are demonstrated. The passivation properties of CdTe/ HgCdTe heterostructures are predicted by modeling the band diagram of abrupt and graded P-CdTe/n-HgCdTe heterostructures. The analysis includes the effect of valence band offset and interface charges on the surface potentials at abrupt hetero-interface, for typical doping levels of the n-type layers and the MOCVD grown CdTe. In the case of graded heterojunctions, the effect of grading on the band diagram for various doping levels is studied, while taking into consideration a generally accepted valence band offset. The MOCVD CdTe with additional pre and post treatments and anneal form the basis of a photodiode with a new design. The new device architecture is based on a combination of a p-on-n homojunction in a single layer of n-type HgCdTe and the CdTe/HgCdTe heterostructure for passivation.

MOCVD growth of ordered Cd(1−x)ZnxTe epilayers

Abstract This work reports for the first time the preparation of ordered Cd (1− x ) Zn x Te epilayers. The epilayers were deposited on CdTe (1xa00xa00) substrates by metal organic chemical vapor deposition in a horizontal quartz reactor. Diethylzinc, dimethylcadmuim and diethyltelluride were used as metalorganic precursors with hydrogen as a carrier gas. Influence of different growth parameters on the growth process is studied for the deposited Cd (1− x ) Zn x Te layers. Transmission electron microscopy diffraction was performed and the long-range ordering effect was observed for the first time, in these layers. The appearance of extra spots attributed to (h± 1 2 , k∓ 1 2 , l± 1 2 ) planes is the indication of CuPt-type atomic ordering in the CdZnTe epilayers.

Single crystalline CdTe solar cells grown by MOCVD

Abstract CdTe has unique advantages for space applications solar cells. Previous structures were not able to yield the full potential of single crystalline solar cells. The limitations of these structures are discussed. We then report a new structure that is being studied with numerical simulations and preliminary experimental results. The actual fabrication and characterization of the cells under study are discussed in detail.

A model for high temperature growth of CdTe by metal organic chemical vapor deposition

A simple kinetic model for the metallorganic chemical vapor deposition (MOCVD) growth of binary epilayers, denoted byAB, is presented. The model yields the growth rate as a function of the gas-phase concentrations of the constituents, in various limiting cases. The model is corroborated with experimental results obtained by the MOCVD growth of CdTe at 480° C. The transition temperature between the surface-reaction and the gasphase mass-transfer control region is shifted to higher temperature with reduced reactor pressure and increased total flow velocity.

A model for the determination of the solid composition of ternary III‐V and II‐VI metalorganic chemical vapor deposition epilayers

A simple kinetic model for the metalorganic chemical vapor deposition growth of ternary III‐V and II‐VI epilayers denoted by AxC1−xB, is presented. The model yields the relationship between the solid composition x of the epilayer and the gas‐phase concentrations of the constituents in various limiting cases. The solid composition x is given by x=(1+α−1×CGMC/CGMA)−1 =[1+α−1(1−Z−1)]−1 where α is a fitting parameter determined by the process parameters, CGMA and CGMC are the gas‐phase concentrations of the metalorganic sources of A and C, respectively. The gas‐phase composition Z is defined by Z=CGMA/(CGMA +CGMC). The predictions of the model are compared with measured data. The data points are calculated with one fitting parameter α, indicating the validity of the model.

Long-range order in CdZnTe epilayers

We report the evidence of long-range order (LRO) in CdZnTe epilayers grown by metal organic chemical vapour deposition (MOCVD) on (100) CdTe. LRO has been characterized by both transmission electron microscopy (TEM) and polarized photoluminescence (PL) measurements. The observation of extra-spots in the diffraction pattern of Cd0.52 Zn0.48 Te samples indicate the CuPt type structure, as corroborated by computer simulation. Valence band splitting (VBS) up to 25 meV, measured by polarized PL gives additional proof of atomic ordering. The order parameter = 0.6 was calculated from VBS using a parabolic approximation.

Characterization of CdTe substrates and MOCVD Cd1−xZnxTe epilayers

Abstract The quality of CdTe substrates and of Cd 1− x Zn x Te ( x ⩽0.1) epilayers grown by metalorganic chemical vapor deposition (MOCVD) on CdTe substrates, were characterized by Raman scattering, by photoluminescence (PL) as well as by X-ray double crystal rocking curve (DCRC). At low temperature the Raman intensity of the LO phonon was enhanced wherever there was a structural defect. The quantitative measure of the structural perfection is related to the ratio between the defect band and excitonic peaks, and correlates with the X-ray full-width at half-maximum (FWHM) of the layer peak. It is shown that in addition to these parameters, the FWHM of the PL defect band is a useful parameter to determine the quality of the epilayer, and a good correlation is obtained between the different parameters. The effect of zinc partial pressure during growth and the reactor design are studied. The results indicate that crystalline imperfection is caused by lattice mismatch between the CdTe substrate and the CdZnTe epilayer and by the nonuniformity of the zinc composition throughout the layers.