Ying‐Lan Chang

Reduced quantum efficiency of a near‐surface quantum well

The effect of the proximity of a bare barrier surface on the quantum efficiency of underlying GaAs/Al0.3Ga0.7As and In0.13Ga0.87As/GaAs quantum wells (QWs) is studied by low‐temperature photoluminescence. The quantum efficiency of the resonantly excited QWs diminishes with decreasing surface barrier thickness; the onset of the reduction in quantum efficiency of the InGaAs QW occurs for a barrier that is 50 A thicker than for the GaAs QW. A simple model of carrier tunneling to the surface is formulated to explain the dependence of the quantum efficiency on surface barrier thickness and well width and height. This model shows good agreement with both sets of experimental data.

Luminescence efficiency of near‐surface quantum wells before and after ion‐gun hydrogenation

We have studied the effects of the proximity of a bare Al0.3Ga0.7As surface on the luminescence of an underlying GaAs quantum well (QW) before and after hydrogenation. The mechanism which is affected by H is tunneling to surface states through the surface barrier. Its thickness was varied by wet etching from 60 to 350 A. Our experiments reveal that the degradation of luminescence efficiency from the QW is dependent on the surface barrier thickness and the excitation energy used in the photoluminescence measurements. A complete recovery or even further enhancement of luminescence efficiency was observed in the near‐surface QW after low‐energy ion‐beam hydrogenation, even at room temperature.

Interaction mechanisms of near‐surface quantum wells with oxidized and H‐passivated AlGaAs surfaces

The tunneling mechanism of electrons and holes to surface states from near‐surface Al0.3Ga0.7As/GaAs quantum wells has been investigated by steady‐state and time‐resolved photoluminescence spectroscopy, near liquid‐helium temperature, of the excitonic e1‐hh1 transition in the well. The ensemble of the data, taken over a wide range of optical excitation levels, for various values of the tunneling‐barrier thickness, and before and after passivation of the surface by hydrogen, allows a description both of the details of the tunneling mechanism and of the character and behavior of relevant surface states. The main results are summarized as follows: (i) steady‐state tunneling is ambipolar, namely, separate for electrons and holes, rather than excitonic; (ii) Spicer’s advanced unified defect model for an oxidized GaAs surface, antisite‐As donors as dominating surface traps, provides an appropriate description of the state distribution at the interface between AlGaAs and its oxide; (iii) hole accumulation in sur...

Long‐term and thermal stability of hydrogen ion‐passivated AlGaAs/GaAs near‐surface quantum wells

Al0.3Ga0.7As/GaAs quantum well samples, irradiated by (100 eV) hydrogen ions at low exposure, have shown improved luminescence for times greater than two years, when stored at room temperature in atmosphere. In situ temperature programmed desorption (TPD) was used to investigate the desorption of AsH3 from an oxidized AlGaAs surface treated by hydrogen ions. Surface modification determined by TPD and Auger electron spectroscopy measurements was correlated with ex situ photoluminescence measurements to further determine the reasons underlying the long‐term room temperature durability of this treatment, as well as the stability of the hydrogen ion passivation at elevated temperatures. Results indicate the importance of the native oxide of the substrate, present during the hydrogenation process, and which may serve as an over‐passivation layer. However, the thermal stability study also indicates that this over‐passivation layer is not thick enough to prevent the degradation of the underlying substrate at ele...

Study of surface stoichiometry and luminescence efficiency of near‐surface quantum wells treated by hydrogen ions and atomic hydrogen

A near‐surface quantum well (QW) structure has been used as an effective probe of surface states before and after different hydrogen treatments. By correlating the surface composition with the luminescence efficiency of the near‐surface QW, we find that the surface passivation is dominated by the defect density of the interface between the AlGaAs surface barrier and the overlying oxide. A complete recovery or further enhancement of luminescence can be readily achieved by treatments using hydrogen ions. However, atomic hydrogen at low exposures is not capable of modifying that interface, and is not effective in passivation. These results corroborate a passivation mechanism which involves removal of As from the interface between the AlGaAs surface barrier and the overlying oxide, and the reduction of interface state density.

STUDY OF HYDROGENATION ON NEAR-SURFACE STRAINED AND UNSTRAINED QUANTUM WELLS

We have studied the effects of hydrogenation on the luminescence efficiency of near‐surface strained InGaAs/GaAs and unstrained GaAs/AlGaAs quantum wells (QWs). By using two different materials with an analogous structure, we have been able to clarify the effects of substrate temperature, ion dosage, strain profile in the material, and material quality on the local hydrogen concentration. This in turn modifies the behavior of hydrogen, the formation of hydrogen‐related defects, and the variation of luminescence efficiency from the near‐surface QW.

Study of temperature dependent hydrogenation on near-surface strained quantum wells

The incorporation of hydrogen by ion‐gun irradiation into near‐surface and deeply embedded In0.13Ga0.87As/GaAs strained quantum wells (QWs) has been studied by photoluminescence spectroscopy and transmission electron microscopy (TEM). Degradation in the free exciton luminescence and the appearance of hydrogen‐related shallow or deep states have been observed within the near‐surface QW after hydrogenation. This effect is more pronounced, the higher the hydrogen dose. In contrast, the deeply embedded QW is only slightly affected by the hydrogenation process even at high substrate temperature and hydrogen ion dose. TEM reveals hydrogen‐induced plateletlike structure in the vicinity of the near‐surface QW and of the GaAs buffer layer/GaAs substrate interface after room temperature and high temperature (250u2009°C) hydrogenation, respectively, which ascertain the extension and nature of the hydrogen‐enriched regions throughout the whole material structure.

Observation of increased photoluminescence decay time in strain‐induced quantum‐well dots

We report on the observation of increased photoluminescence (PL) decay time in strain‐induced quantum‐well dots (SIQWDs), 120 nm in diameter. Lateral confinement was generated in a GaAs quantum well (QW) by etching a doubly exposed grating pattern into a pseudomorphic, strained layer of In0.35Ga0.65As which overlies the QWs. By spacing three QWs of different widths at varying depths from the In0.35Ga0.65As stressor, lateral strain confinement and vertical strain propagation are directly resolved by the PL spectra. The decay time of the heavy‐hole‐like excitons in the SIQWDs from the uppermost QW is 420 ps at 2 K, which is longer than the 270 ps PL decay time of the heavy‐hole exciton in the reference QW sample.

Passivation of InGaAs/InP surface quantum wells by ion‐gun hydrogenation

We have investigated the optical properties of an InGaAs/InP surface quantum well before and after room‐temperature low‐energy ion‐gun hydrogenation. The luminescence efficiency of the surface quantum well was enhanced by up to two orders of magnitude after hydrogenation. Our experiments also reveal that the nonradiative recombination centers at the etched surface can be saturated by increasing excitation density for the photoluminescence measurement. To ‘‘unmask’’ the effects of the saturation of recombination sites, for a true comparison of passivation effects brought about by different surface treatments, an excitation density below 1 W/cm2 is required.

Fusion bonding : hetero-interfacial materials analysis and device application

A large number of novel devices have been recently demonstrated using wafer fusion to integrate materials with different lattice constants. In many cases, devices created using this technique have shown dramatic improvements over those which maintain a single lattice constant. We present device results and characterizations of the fused interface between several groups of materials.