K. P. Killeen

Low resistance wavelength‐reproducible p‐type (Al,Ga)As distributed Bragg reflectors grown by molecular beam epitaxy

We report the reproducible molecular beam epitaxial growth of Be‐doped piecewise linearly graded (Al,Ga)As distributed Bragg reflectors that have vertical series resistivities near bulk values. For mirrors with three linear segments per interface, the center wavelength reproducibility is 0.1% and the series resistivity is as low as 1.8×10−5 Ω cm2 for hole concentrations of 5×1018 cm−3. Measured reflectivities of 6.0% per interface are comparable to conventional single‐linear‐grade mirrors. Vertical‐cavity surface‐emitting lasers incorporating these mirrors exhibit record‐low voltage thresholds of less than 1.5 V.

Method for accurate growth of vertical-cavity surface-emitting lasers

We report a method for accurate growth of vertical-cavity surface-emitting lasers (VCSELs). The method uses a single reflectivity spectrum measurement to determine the structure of the partially completed VCSEL at a critical point of growth. This information, along with the extracted growth rates, allows imprecisions in growth parameters to be compensated for during growth of the remaining structure, which can then be completed with very accurate critical dimensions. Using this method, we can now routinely grow lasing VCSELs with Fabry-Perot cavity resonance wavelengths controlled to within 0.5%.

Real-time control of molecular beam epitaxy by optical-based flux monitoring

We have developed a real‐time molecular beam epitaxy control system based on Al and Ga atomic beam resonant absorption of hollow cathode lamp emission. By continuously monitoring the absorption of Al and Ga beams during growth, this system accurately determines instantaneous growth rates and then integrates these over time to determine layer thickness. This information is used in real time for effusion cell shutter (and hence layer thickness) control. The accuracy and flexibility of this system is demonstrated here by growing AlAs/GaAs distributed Bragg reflectors with consistent layer thicknesses even though effusion cell temperatures were intentionally varied. In each instance the system automatically detected and compensated for the different growth rates, resulting in DBRs with center wavelengths controlled to within 0.3% of the target wavelength.

Accurate multiple-quantum-well growth using real-time optical flux monitoring

We report a real‐time molecular beam epitaxy control system based on optical flux monitoring (OFM) that is capable of producing thin AlAs/GaAs layers of accurate thickness. We demonstrate the system’s ability to detect and compensate for growth rate variations by growing AlAs/GaAs multi‐quantum‐well structures while deliberately ramping the GaAs growth rate to simulate a severe effusion cell instability. Results show that a sample grown under these conditions without OFM control (i.e., while using conventional timed shutter control) exhibited multiple photoluminescence peaks, indicating that its quantum wells differed in thickness, while a sample grown using OFM shutter control exhibited a single narrow peak, indicating that its quantum wells were nearly identical in width. Analysis of the OFM shutter control sample’s photoluminescence linewidth shows that the resulting quantum‐well thickness variation were less than 1%.

Reactive chemical intermediates in metalorganic chemical vapor deposition of GaAs

Reactive chemical gas‐phase intermediate species important in the metalorganic chemical vapor deposition growth process of GaAs have been identified for the first time using in situ ultraviolet absorption spectroscopy. Spectral features from Ga, GaH, and GaCH3 were observed during thermal decomposition of trimethylgallium (TMGa) in hydrogen gas. Evidence for free radical reactions involving CH3 and H in the pyrolysis of TMGa is presented. The addition of arsine to the mixture is found to change the reaction pathways as evident from the production of AsH2 and the disappearance of Ga and GaH. Gas‐phase decomposition of the metalorganic species in hydrogen can produce gallium under the conditions of high pressure atomic layer epitaxy.

Excitation of the HgBr B/sup 2/. sigma. /sup +//sub 1/2/reverse arrowX/sup 2/. sigma. /sup +//sub 1/2/ band in the ultraviolet

The buildup and decay of the HgBr X2Σ1/2+ species during a pulsed discharge in Ne, N2, and HgBr2 vapor and in its afterglow have been examined by monitoring B2Σ1/2+←X2Σ1/2+ absorption by the discharge at 351 nm. Also, the UV-to-green conversion efficiency for the 350-nm pumped HgBr laser is shown to be sensitive to the 502-nm fluence circulating in the cavity when the XeF laser pump pulse arrives. The maximum observed efficiency (η) of 22% is obtained for intracavity intensities of 1 to 2 MW/cm2, but η falls to ~10% for I ≃ 1 kW/cm2.The buildup and decay of the HgBr X/sup 2/..sigma../sup +//sub 1/2/ species during a pulsed discharge in Ne, N/sub 2/, and HgBr/sub 2/ vapor and in its afterglow have been examined by monitoring B/sup 2/..sigma../sup +//sub 1/2/reverse arrowX/sup 2/..sigma../sup +//sub 1/2/ absorption by the discharge at 351 nm. Also, the UV-to-green conversion efficiency for the 350-nm pumped HgBr laser is shown to be sensitive to the 502-nm fluence circulating in the cavity when the XeF laser pump pulse arrives. The maximum observed efficiency (eta) of 22% is obtained for intracavity intensities of 1 to 2 MW/cm/sup 2/, but eta falls to --10% for Iapprox. =1 kW/cm/sup 2/.

Metalorganic Vapor Phase Epitaxial Growth Of Red And Infrared Vertical-cavity Surface-emitting Laser Diodes

Vertical-cavi ty surface-emitting lasers (VCSELs) are destined to find widespread application due to their many unique advantages, including single longitudinal optical mode operation, surface-normal output, ease of fabrication into two-dimensional arrays and wafer-level testing. Such devices have also pushed the limits of conventional crystal growth techniques due to complex and demanding epitaxial requirements. Besides early pioneering work on VCSELs grown in part using MOVPE [l], molecular-beam epitaxy (MBE) has long been regarded as the preferred growth technique for all-epitaxial AlGaAs-based VCSELs, due in part to readily available in-situ diagnostic techniques. We have investigated the use of MOVPE for several potential advantages in VCSEL growth, including integration of different materials systems such as the phosphides for a wider operational wavelength range, relative ease of compositional grading for low-resistance distributed Bragg reflectors (DBRs), a variety of dopant choices including carbon, higher growth rates in thick device structures, and improved surface morphology. This work has now led to the demonstration of record performance levels in InGaAdAlGaAs-based IR (960 nm) VCSELs employing extensive continuous grading schemes in the DBRs, and efficient room-temperature cw operation of VCSELs operating at red (670 nm) wavelengths, using AlGaInPIAlGaAs hetero-structures in the optical cavity.