We employ an alternate approach to Stranski-Krastanow (SK) QD formation involving the use of nanopatterning with
diblock copolymers combined with selective MOCVD growth, enabling QD formation over large surface areas intended
for device application. This approach allows for increased control over the QD size and distribution and elimination of
the problematic wetting layer associated with SK QDs. Cross-sectional TEM studies of the nanopatterned QD active
regions confirm the absence of a wetting layer, and AFM/SEM measurements indicate high QD densities are achieved
(>6x1010 cm-2). Furthermore, the process is applicable to large surface coverage, showing promise for implementation
into long wavelength (λ = 1.3-1.5μm) sources employing either lattice-matched or strained QDs. Preliminary device
results demonstrate LT (up to 170K) InP-based laser operation from devices employing patterned lattice-matched InxGa1-
xAs QD (~ 20 nm dia.) active regions. The formation of high density compressively strained InAs QDs on InP substrates
is also demonstrated using the nanopatterning and selective growth process.
The optical spectral gain characteristics and overall radiative efficiency of MOCVD grown InGaAs quantum dot lasers
have been evaluated. Single-pass, multi-segmented amplified spontaneous emission measurements are used to obtain the
gain, absorption, and spontaneous emission spectra in real units. Integration of the calibrated spontaneous emission
spectra then allows for determining the overall radiative efficiency, which gives important insights into the role which
nonradiative recombination plays in the active region under study. We use single pass, multi-segmented edge-emitting in
which electrically isolated segments allow to vary the length of a pumped region. In this study we used 8 section devices
(the size of a segment is 50x300 μm) with only the first 5 segments used for varying the pump length. The remaining
unpumped segments and scribed back facet minimize round trip feedback. Measured gain spectra for different pump
currents allow for extraction of the peak gain vs. current density, which is fitted to a logarithmic dependence and directly
compared to conventional cavity length analysis, (CLA). The extracted spontaneous emission spectrum is calibrated and
integrated over all frequencies and modes to obtain total spontaneous radiation current density and radiative efficiency,
ηr. We find ηr values of approximately 17% at RT for 5 stack QD active regions. By contrast, high performance InGaAs
QW lasers exhibit ηr ~50% at RT.
The conventional approach to fabricate semiconductor based QDs is based on the Stranski-Krastnow (SK) growth mode,
which has enjoyed considerable success in device applications. However, the SK QD approach is complicated by the
randomness of the QD size distribution and inherent presence of the wetting layer. Carrier leakage to the wetting layer
has been identified as one of the underlying causes for low optical gain and high temperature sensitivity in diode lasers.
To fully exploit the potential advantages of ideal Quantum Dots (i.e. full 3D carrier confinement), elimination of the
wetting layer and a uniform mono-modal QD size distribution is needed. Nanopatterning with selective MOCVD QD
growth has potential for achieving a higher degree of control over the QD formation, compared with the SK process.
Furthermore, the problematic wetting layer states are eliminated and improved optical gain is expected. The QD
patterning is prepared by dense nanoscale (20-30 nm diameter) diblock copolymer lithography, which consists of
perpendicularly ordered cylindrical domains of polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) matrix. For
selective MOCVD growth, a dielectric template mask was utilized and the polymer patterning is transferred on it. The
resulting GaAs QD densities are larger than 5×1010/cm2, comparable to SK growth mode, with a nearly monomodal QD
size distribution. Variable temperature PL has been used to characterize the optical properties of capped InGaAs QDs on
GaAs (λ ~ 1.1 μm) and InP (λ ~ 1.5 μm) substrates.
As an alternate Quantum Dot (QD) fabrication method to self-assembled SK mode QDs, diblock copolymer nano-patterned QDs were investigated. By employing selective growth of QDs on diblock copolymer nano-patterned masks, independence from the problematic wetting layer and controllability on QD size and distribution associated with SK growth mode QDs were realized. The diblock copolymer nano-patterned masks were fabricated using a diblock copolymer template and a dielectric mask, and InxGa1-xAs QDs were selectively grown on patterned GaAs and InP substrates by Metalorganic Chemical Vapor Deposition (MOCVD). The optical properties from diblock copolymer patterned QDs on III-V substrates were investigated at low temperature.