Peter Y. Yu

Optical investigation of defects in AgGaS2 and CuGaS2

Abstract Defects in two chalcopyrite semiconductors AgGaS 2 and CuGaS 2 have been studied by a combination of several optical techniques including absorption, emission, Raman scattering and their excitation spectroscopies. From these measurements we have determined the binding energies of both shallow and deep donors and acceptors in the two materials.

Pressure Dependence of Band Gaps in the Quaternary Semiconductors Cu(In, Ga)Se2

We report the photoluminescence and absorption spectra of a series of quaternary chalcopyrite alloys with the formula: CuIn1-xGaxSe2 where x varies between 0 and 1 in steps of 0.25. Using a diamond anvil cell we have studied their band gaps and impurity emission peak energies as a function of pressure. We found that while the band gap of CuIn1-xGaxSe2 varies nonlinearly with x the pressure coefficient (alpha) varies linearly with x at a rather large rate of d alpha/dx = 18.6 meV/GPa. We interpret our results in terms of recent theoretical calculations which have been proposed to explain the large difference in alpha between CuGaSe2 and CuInSe2.

Pressure dependence of Raman modes in the chalcopyrite quaternary alloy AgxCu1-xGaS2

PRESSURE DEPENDENCE OF RAMAN MODES IN THE CHALCOPYRITE QUATERNARY ALLOY Ag x Cu 1-x GaS 2 . In-Hwan Choi Department of Physics, Chung-Ang University, Seoul, Korea and Peter Y. Yu (a) Department of Physics, University of California, Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. ABSTRACT Raman scattering in the chalcopyrite quaternary alloy Ag x Cu 1-x GaS 2 has been studied under high pressure (up to 7 GPa) and at low temperature (50 K) using a diamond anvil high pressure cell for alloy concentrations x=1, 0.75, 0.5, 0.25 and 0. This has allowed us to determine the dependence of their zone-center phonon modes on both pressure and alloy concentration. The resultant phonon pressure coefficients are helpful in understanding the nature of the phonon modes in these chalcopyrites. PACS:78.55.Hx; 78.40.-q; 78.20.Hp Substance Classification: S8.12 (a) email: PYYU@LBL.GOV; fax: 510-643-8497.

Deep levels in AgGaSe2

Abstract The photoluminescence spectra of AgGaSe 2 obtained at 77 K are reported. Two emission peaks have been observed and their pressure dependence studied. One of these peaks has been identified with emission involving one of the two deep-level peaks, D 1 and D 2 observed previously in absorption measurement [ Appl. Phys. Lett. 64, 1717 (1994)]. A simple model for the capture of carriers into these deep levels has been proposed.

Experimental and theoretical studies on gadolinium doping in ZnTe

We studied the effects of Gd doping on the structural and optical properties of ZnTe films grown by pulsed laser deposition. We found that a small amount of Gd doping yields a reduction in the ZnTe lattice constant with no change in the fundamental band gap of the material. When the doping level increases above 7% the lattice constant becomes more or less constant, while the band gap increases abruptly (by as much as 50 meV). Theoretical calculations based on ZnTe supercells containing either isolated defects or defect complexes show that the reduced lattice constant can be attributed to the presence of defect complexes involving substitutional Gd ions and neighboring vacancies. The insensitivity of the band gap to low Gd concentration can be explained by self-compensation of these defects, while the band-filling effect probably explains the increase in the band gap energy.

Light Scattering Spectroscopies of Semiconductor Nanocrystals (Quantum Dots)

We review the study of nanocrystals or quantum dots using inelastic light scattering spectroscopies. In particular recent calculations of the phonon density of states and low frequency Raman spectra in Ge nanocrystals are presented for comparison with experimental results.

A New Microscopic Theory of Low Frequency Raman Modes in Ge Nanocrystals

A bond polarizability model in combination with a lattice dynamics model has been used to compute the intensities and frequencies of low frequency Raman modes reported in Ge nanocrystals. The results indicate that the cross‐polarized mode often identified as a torsional Lamb mode (which is Raman‐inactive) is actually a l=2 spheroidal mode.

Pressure dependence of defect emissions and the appearance of pressure-induced deep centers in chalcopyrite alloys AgxCu1-xGaS2

PRESSURE DEPENDENCE OF DEFECT EMISSIONS AND THE APPEARANCE OF PRESSURE-INDUCED DEEP CENTERS IN CHALCOPYRITE ALLOYS Ag x Cu 1-x GaS 2 In-Hwan Choi Department of Physics, Chung-Ang University, Seoul, Korea and Peter Y. Yu (a) Department of Physics, University of California, Berkeley, and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. ABSTRACT We present the pressure dependence of the defect emissions in the chalcopyrite alloy semiconductor Ag x Cu 1-x GaS 2 for values of the alloy concentration x varying between 0 and 1. A large variation in the pressure coefficients of the different defect emissions with x was found. In one alloy concentration x=0.25 deep levels were found to appear under pressure. Plausible explanations of our results have been proposed. PACS:78.20.Hp; 78.30.Hv; 63.20.Kr Substance Classification: S8.11 (a) email: PYYU@LBL.GOV; fax: 510-643-8497.

A Density Functional Theory Study of Ferromagnetism in GaN:Gd

First principle calculations of the electronic structure and magnetic interaction of GaN:Gd have shown that the ferromagnetic p–d coupling is over two orders of magnitude larger than the s–d exchange coupling. The experimental room temperature ferromagnetism in GaN:Gd are explained by the interaction of Gd 4f spins via p–d coupling involving localized holes introduced by intrinsic defects such as Ga vacancies.

Chapter 5.2 High-Pressure Study of DX Centers Using Capacitance Techniques

Publisher Summary This chapter discusses high-pressure study of DX centers using capacitance techniques. Pressure can change the band structure of a semiconductor without changing its symmetry or composition. Thus, pressure is a powerful technique for studying the influence of electronic band structures on the properties of defects in semiconductors. Defect centers in semiconductors are usually classified as shallow (or hydrogenic) and deep. A defect energy level whose wave function can be constructed out of the near-band extremum is considered shallow. On the other hand, the wave function of a highly localized center can be expressed only as a linear combination of wave functions from a large region of the Brillouin zone. The properties of shallow and deep centers are quite different—for example, their pressure dependence. The chapter describes the way of performing capacitance measurements on samples subjected to high pressure inside the diamond anvil cell (DAC).