D. Goren

Measurement of band offsets and interface charges by the C–V matching method

The present article describes a novel application of capacitance–voltage measurements to determine simultaneously the band discontinuities (ΔEV, ΔEC) and interface charge density (σ) of heterojunctions. The method, which we refer to as C–V matching, complements the most versatile C–V profiling technique proposed by Kroemer and successfully applied by others. In contrast to the C–V profiling which is limited to isotype heterojunctions, the new method is applicable to p-n heterojunctions as well. The methodology is based on three cardinal equations which are not controversial—the lineup of the bands relative to the common Fermi level (at equilibrium) or the quasi-Fermi levels (when voltage is applied), the charge neutrality and the expression for the total capacitance of the heterostructure. The three equations are formulated for equilibrium as well as nonequilibrium conditions, using quasi-Fermi levels and the quasi-equilibrium approximation. The three cardinal equations are defined by the two constant (al...

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.

Shockley–Read recombination and trapping in p‐type HgCdTe

The concepts and definitions of the steady‐state minority‐carrier lifetime, the steady‐state majority‐carrier lifetime, and the transient excess‐carrier lifetime in semiconductors are reviewed. The effects that Shockley–Read centers have on these lifetimes are discussed, with emphasis given to the case of p‐type Hg0.775Cd0.225Te containing traps. Measurements of the excess‐carrier lifetime in Hg0.775Cd0.225Te (Na∼1×1016/cm3) using various experimental techniques are then summarized in view of the above definitions.

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.

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.

Determination of interface properties between a depleted heteroepitaxial layer and a substrate from capacitance measurements

A method for the determination of interface properties between a depleted heteroepitaxial layer and a substrate is presented. The method is based on the measurement of the zero bias capacitance of a metal gate that forms a Schottky contact with the depleted epilayer. The interface charge density can be extracted regardless of the exact values of the energy band discontinuities. A microscopic electrostatic analysis of the interface is presented which takes into account the effects of interface charges and interface dipoles. Experimental results are presented for the case of ZnTe epilayers grown by metal organic chemical vapor deposition on CdTe substrates.

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.

Barrier formation at graded HgTe/CdTe heterojunctions

Numerical calculations of graded HgTe/CdTe heterojunction (HJ) band diagrams at equilibrium are presented and discussed. The calculations are performed in the entire compositional range (0<x<1), using a nonparabolic conduction band and Fermi‐Dirac statistics. The dependence of barrier formation at graded HJs are examined as a function of the graded region width and the graded region doping profiles. The graded region width and doping profiles were found to be the two main factors that determine whether barriers are formed as well as their shape and magnitude. The calculated results indicate that epitaxial ohmic HgTe contacts to extrinsic CdTe are possible, provided that the graded region is wider than one micron, and that it has the same doping type as the doping of the substrate with equal or higher absolute value. Further numerical calculations take into consideration the possible existence of distributed interface charges in the graded region of the HJ. It is shown that by assuming a classical transpor...

Band diagram of a HgTe‐CdTe semimetal‐semiconductor abrupt heterostructure

The energy‐band diagram and interface potentials at a HgTe‐CdTe abrupt heterostructure are presented. An analytic approximation is formulated and compared to an exact (numeric) calculation of the interface potentials. The error introduced by the analytic approximation is negligible for a wide range of doping levels of the CdTe. The analysis predicts that HgTe forms ohmic contacts to p‐type CdTe and rectifying junctions to n‐type CdTe. Interface charges above 1012 cm−2 modify the interface potentials. Positive interface charge imposes depletion in p‐type CdTe resulting in potential barriers that may degrade the ohmic properties of the contacts. The methodology presented in this study may be extended to additional semimetal‐semiconductor heterostructures.

Photocurrent in CdTe NIP solar cells

Abstract A simple analytical model for the voltage dependence of the photocurrent in CdTe n–i–p solar cells is presented. The physical model is corroborated with a numerical solution of Poisson and the two continuity equations under illumination and shows excellent agreement with the numerical data. The new model is compared with previously reported models of Bube and Crandall. The new model illuminates the loss mechanism of carriers near the front interface of CdTe solar cells. It is shown that the photocurrent is high when the carrier velocity due to the electric field is higher than the product of the absorption coefficient and the diffusion coefficient, almost regardless of the interface recombination velocity value. At lower electric field values, the interface recombination velocity has a stronger effect on the photocurrent. Experimental conditions leading to a reduction of the electric field and a corresponding decrease in the collection efficiency in CdTe n–i–p solar cells are discussed.