T-shaped quantum-wire laser 2004 Fall MRS meeting in Boston (2004.11.30 B3.1) T-shaped quantum-wire laser M. Yoshita, Y. Hayamizu, Y. Takahashi, H. Itoh, T. Ihara, and H. Akiyama Institute for Solid State Physics, Univ. of Tokyo and CREST, JST L. N. Pfeiffer, K. W. West, and Ibo Matthews Bell Laboratories, Lucent Technologies 1. Formation of high-quality GaAs T-shaped quantum wires Cleaved-edge overgrowth with MBE, AFM, PL, PLE 2. Single-wire laser PL scan, Lasing, PL, Absorption/Gain via Cassidy’s method, Transmission 3. Observation of RT 1D exciton absorption in 20-wire laser 4. Optical response of n-doped single-wire FET device

T-shaped Quantum Wire (T-wire) T-shaped QWR, or T-wire, that I talk today, has a crosssectional structure shown here. A QWR is formed at a T-intersection of two QWs, a stem QW and an arm QW with thickness of 14nm and 6nm. Since quantum confinement is weaker on a low barrier than on a high barriers, 1-D state is formed quantum mechanically at the intersection. In fact the red and blue contour curves show electron and hole wavefunctions in a T-wire exciton.

Cleave in situ 600oC 490oC (490oC) Cleaved-edge overgrowth with MBE by L. N. Pfeiffer et al., APL 56, 1679 (1990). [110] [001] GaAs substrate T-wires are formed by cleaved-edge overgrowth method with MBE developed by Loren Pfeiffer in Bell Labs. We first make a stem quantum well by conventional MBE growth on a (001) substrate at 600C.The wafer is once taken out from the MBE machine, thinned, cut into pieces, partly scribed, and then reloaded into the machine. After setting growth condition, we cleave the sample, and grow an arm well right away. There are two major difficulties. The first is in-situ cleavage at growth temperature. The second difficulty is the arm well growth. If you cleave you always have (110) surface, and the MBE growth on (110) surface is difficult. You need low temperature of about 500C, and high As pressure for (110) MBE. (001) MBE Growth Cleave in situ (110) MBE Growth 600oC 490oC (490oC)

Nomarski Microscope Images of Cleaved-Edge- Good Overgrowth Surfaces Poor Bad Nomarski Microscope Images of Cleaved-Edge- Overgrowth Surfaces “Hackling”

High Quality T-wire ??? 490C Growth Atomically flat interfaces With this annealing technique, we are now able to fabricate very high quality T-wires. All previous T-wires had a rough top interface on an arm well. But we now have an atomically flat top interface by annealing, and also an flat bottom interface by cleavage, and remaining interfaces can be optimized in the conventional MBE growth. Before we apply this technique to real T-wires, I have a comment. If the amount of deposited film thickness is exactly integer monolayers, we get this flat surface. But if we supply it a little too much or too less, we should not obtain this flat surface. Atomically flat interfaces (By Yoshita et al. JJAP 2001)

High Quality T-wire !? 490C Growth 510-600C Anneal Atomically flat With this annealing technique, we are now able to fabricate very high quality T-wires. All previous T-wires had a rough top interface on an arm well. But we now have an atomically flat top interface by annealing, and also an flat bottom interface by cleavage, and remaining interfaces can be optimized in the conventional MBE growth. Before we apply this technique to real T-wires, I have a comment. If the amount of deposited film thickness is exactly integer monolayers, we get this flat surface. But if we supply it a little too much or too less, we should not obtain this flat surface. Atomically flat interfaces (By Yoshita et al. JJAP 2001)

High Quality T-wire !!? 490C Growth 510-600C Anneal Atomically flat With this annealing technique, we are now able to fabricate very high quality T-wires. All previous T-wires had a rough top interface on an arm well. But we now have an atomically flat top interface by annealing, and also an flat bottom interface by cleavage, and remaining interfaces can be optimized in the conventional MBE growth. Before we apply this technique to real T-wires, I have a comment. If the amount of deposited film thickness is exactly integer monolayers, we get this flat surface. But if we supply it a little too much or too less, we should not obtain this flat surface. Atomically flat interfaces (By Yoshita et al. JJAP 2001)

High Quality T-wire !!!!! 490C Growth 510-600C Anneal Atomically flat With this annealing technique, we are now able to fabricate very high quality T-wires. All previous T-wires had a rough top interface on an arm well. But we now have an atomically flat top interface by annealing, and also an flat bottom interface by cleavage, and remaining interfaces can be optimized in the conventional MBE growth. Before we apply this technique to real T-wires, I have a comment. If the amount of deposited film thickness is exactly integer monolayers, we get this flat surface. But if we supply it a little too much or too less, we should not obtain this flat surface. Atomically flat interfaces (By Yoshita et al. JJAP 2001)

20-wire laser sample 14nm x 6nm 1st growth 600C 2nd growth 490C (arm well) 600C 10min anneal 490C (cover layers) This shows the schematic structure of our 20-wire laser. 20 wires with the size of 14nm by 6nm are formed in T-shaped optical waveguide. After the growth of the arm well, we introduced the growth interrupt anneal at 600 degree C for 10 minutes. We obtained 500micron-long laser bars by cleavage. The cleaved facet mirrors are left uncoated. We cooled down a laser bar to 5K and measured PL from the top with a microscope or measured stimulated emission coming out from a cavity mirror facet. Laser bars 500mm uncoated cavity

PL and PLE spectra Sharp PL width Small Stokes shift 1D free exciton 1D continuum states arm well T-wire stem well (Akiyama et al. APL 2003)

E-field E-field // to wire _ to wire // to arm well I 点励起。

Single quantum wire laser Probability of Photon Cavity length　500 mm G=5x10-4 In this work, we used the single quantum wire laser. This is the schematic view. Probability of Electron

Scanning micro-PL spectra T-wire stem well T-wire stem well T=5K ContinuousPL peak over 20 mm PL width < 1.3 meV We characterized our sample by scanning micro-PL spectra for 1 micro m spot along the quantum wire. These were measured by 10 micro m steps for 500 micro m region. And for this 25 micro m region we made scan by finer 0.5 micro m steps. The high energy peaks are from stem well and the low energy peaks are from a quantum wire. (Arm well peak was not observed at this low pumping level.) These tiny low energy peaks of wire are attributed to localized excitons caused by monolayer fluctuation of the arm well thickness. In this region, there is no tiny peak of localized excitons, and this shows that the region without monolayer fluctuation extends over 20 micro m. This results prove the high quality of this quantum wire.

Lasing in a single quantum wire 500mm gold-coated cavity Threshold 5mW (Hayamizu et al, APL 2002)

Excitation power dependence of PL M. Yoshita, et al.

Density Electron-hole Plasma Biexciton+Exciton Free Exciton n1D = 1.2 x 106 cm-1 (rs = 0.65 aB) aB ~13nm Electron-hole Plasma Density n1D = 1.2 x 105 cm-1 (rs = 6.6 aB) Biexciton+Exciton EB =2.8meV n1D = 3.6 x 103 cm-1 (rs = 220 aB) Free Exciton n1D ~ 102 cm-1

Absorption/gain measurement based on Hakki-Paoli-Cassidy’s analysis of Fabry-Perot-laser emission below threshold ：Absorption coeff. The waveguide emission spectra are represented by this equation. In the Cassidy’s method, we use the total intensity of the longitudinal modes peak “I sum” and minimal intensity “I min”. By using the ratio of I sum and I min, p, the absorption coefficients of each longitudinal mode are derived analytically by this equation. ：Reflectivity D. T. Cassidy JAP. 56 3096 (1984) Free Spectral Range B. W. Hakki and T. L. Paoli JAP. 46 1299 (1974)

Absorption Spectrum by Cassidy method Single wire laser, uncoated cavity mirrors Excitation Light ：cw TiS laser at 1.631eV Spectrometer with spectral resolution of 0.15 meV Cassidy’s Method Point Waveguide Emission By using the Cassidy’s method, we can derive the absorption spectrum from the waveguide emission spectrum. This absorption spectrum shows the several peaks: exciton ground state, 1st excited state and continuum state. These tiny peaks are attributed to the localized excitons caused by monolayer terrace of the arm well. This spectra shows good agreement with the result of photo luminescence excitation spectrum. Polarization parallel to Arm well

Measurement of absorption/gain spectrum Excitation Light ：cw TiS laser at 1.631eV Spontaneous emission Spectrometer with spectral resolution of 0.15 meV Stripe shape Cassidy’s Method Waveguide Emission This blue curve is the derived absorption spectrum of the single quantum wire laser by using Cassidy’s method at the high excitation power. In this figure, the absorption is positive, the gain is negative. The absorption spectrum has a continuous absorption band and a symmetric gain peak. The energy of the gain peak corresponds to the shoulder of the broad PL peak, and also the lasing peak. This absorption spectrum could be explain by the two band model. The gain is yielded by the population inversion of the electron-hole plasma. We can attribute here to the band edge energy, and here to the Fermi edge energy. To investigate the gain peak in more detail, we measured the 20 wires laser. Each wire has same size as previous single quantum wire. 8.3mW Polarization parallel to Arm well

Absorption/gain spectrum (High excitation power) Electron-Hole Plasma EFE EBE Gain Absorption This blue curve is the derived absorption spectrum of the single quantum wire laser by using Cassidy’s method at the high excitation power. In this figure, the absorption is positive, the gain is negative. The absorption spectrum has a continuous absorption band and a symmetric gain peak. The energy of the gain peak corresponds to the shoulder of the broad PL peak, and also the lasing peak. This absorption spectrum could be explain by the two band model. The gain is yielded by the population inversion of the electron-hole plasma. We can attribute here to the band edge energy, and here to the Fermi edge energy. To investigate the gain peak in more detail, we measured the 20 wires laser. Each wire has same size as previous single quantum wire. 8.3mW Hayamizu et al.

Electron-Hole Plasma Exciton Exciton peak and continuum onset decay without shift. Gap between exciton and continuum is gradually filled. Exciton changes to Fermi edge Exciton Hayamizu et al.

Transmission measurement of a single quantum wire ~mm ~nm

Transmittance for single Coupling efficiency = 20% Takahashi et al. unpublished

Takahashi et al. unpublished Absorption for single Takahashi et al. unpublished

Absorption for 20 Y. Takahashi et al.

Room-Temperature 1D Exciton Absorption! Absorption at 300 K Room-Temperature 1D Exciton Absorption! Y. Takahashi et al.

14nmx6nm Doped Single Wire FET device with tunable 1D electron density T. Ihara et al.

Quantum-Wire Devices

Summary GaAs T-shaped quantum wires (T-wires) are formed by cleaved-edge overgrowth with MBE. Growth-interrupt anneals dramatically improve T-wire quality. AFM : No atomic steps over 100mm. PL : Sharp PL width (~1meV) improved by a factor of 10. PLE : Observation of 1D free exciton, & 1D continuum states Single wire lasing : The world thinnest laser (14nm x 6nm), 5mW threshold optical pumping power at 5K. Gain/absorption measurement by Hakki-Paoli-Cassidy’s method. Strong photo-absorption by a single wire 84/cm (98.5% absorption / 500mm) at exciton peak at 5K Room-temperature exciton absorption observed in 20-wire laser. Single-wire FET: carrier-sensitive optical responses.

ここまで。25分のトーク。

(001) and (110) surfaces of GaAs [001] [110] [110] [001]

Growth rate of GaAs in MBE holder Substrate As Ga Substrate rotation >>> uniform holder Substrate Ga limited growth under As4　overpressure As Ga

Interface control by growth-interruption annealing 490oC Growth (by M. Yoshita et al. JJAP 2001) Atomically flat interfaces 600oC Anneal High Quality T-wire arm well 6nm stem well 14nm

(By Yoshita et al. APL 2002)

Single wire laser with 500mm gold-coated cavity

Absorption at higher temperatures by Cassidy 前の図との共通点を述べる。 Hayamizu et al. unpublished

Absorption coefficients

Experiment for gain

Evolution of continuum Takahashi et al. unpublished

Lasing & many-body effects in quantum wires E. Kapon et al. (PRL’89) Lasing in excited-states of V-wires W. Wegscheider et al. Lasing in the ground-state of T-wires, no energy shift, (PRL’93) excitonic lasing R. Ambigapathy et al. PL without BGR, strong excitonic effect in V-wires (PRL’97) L. Sirigu et al. (PRB’00) Lasing due to localized excitons in V-wires J. Rubio et al. (SSC’01) Lasing observed with e–h plasma emission in T-wires A. Crottini et al. (SSC’02) PL from exciton molecules (bi-excitons) in V-wires T. Guillet et al. (PRB’03) PL, Mott transition form excitons to a plasma in V-wires H. Akiyama et al. Lasing due to e–h plasma, no exciton lasing in T-wires (PRB’03) F. Rossi and E. Molinari (PRL’96) F. Tassone, C. Piermarocchi, et al. (PRL’99,SSC’99) S. Das Sarma and D. W. Wang (PRL’00,PRB’01) Theories “1D exciton Mott transition”

Physical picture of 1D exciton–plasma transition the exciton Mott transition Our PL results show Increase of e–h pair density causes no energy shift of the exciton band edge ・ reduction of exciton binding energy ・ red shift of the band edge (band-gap renormalization (BGR)) plasma low-energy edges appear at the bi-exciton energy positions, and show BGR no connection, but coexistence of two band edges eg. D. W. Wang and S. Das Sarma, PRB 64, 195313 (2001). band edge most recent theory this evolution is treated by the Mott transiion no level-crossing between the band edges and the exciton level exciton level

Exciton band edge & plasma band edge (T=30K) (low energy edge of plasma PL) starts at biexciton energy and shows red shift. ▼ exciton band edge, (onset of continuum states) exciton ground and excited states show no shift.

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Electron plasma and minority hole e e e e h e e e e e e h e X- Charged Exciton X Exciton e h

1D exciton and continuum states Theory 1D exciton and continuum states