GaN laser with refractory metal ELOG masks for intracavity contact

Refractory metal ELOG mask are used for GaN based VCSELs and edge emitter structures to serve as intracavity contacts. In these structures the refractory metal ELOG masks serve both as ohmic contact metals as well as masks for ELOG.

BACKGROUND

An issue in fabricating GaN and other nitride based lasers concerns the high-resistance intra-cavity contacts that are typically formed with nitride semiconductor material. This typically arises due to the poor p-type conductivity of GaN and typically requires that the metal contacts be placed close to the active region to reduce heating and voltage drops. Typically involved etching process are required to place the metal contacts in the required locations of the GaN lasers.

SUMMARY

Refractory metal masks are used in accordance with the invention with an epitaxial layer overgrowth process (ELOG) and positioned relative to the laser active region to provide intracavity contacts and such that the refractory metal masks introduce minimal optical absorption loss. Refractory metal masks are used in place of SiO2or Si3N4masks for selective ELOG and also function as ohmic contact metals.

DETAILED DESCRIPTION

Typically GaN edge emitting laser structure100is made by taking substrate105, typically Al2O3or SiC and depositing GaN buffer layer110over it to a typical thickness of about 30 nm. Then planar n-type GaN layer120is deposited over GaN buffer layer110as shown inFIG. 2a.Refractory metal ELOG mask130is deposited by sputtering or evaporation and patterned by chemically assisted ion beam etching (CAIBE) or reactive ion-etching (RIE) over n-GaN layer120. Then ELOG growth is started using refractory metal ELOG mask130for growing n-GaN layer138to a typical thickness of about 1 to about 2 μm; n-AlGaN lower cladding layer140has a typical thickness of about 1 μm; active region199which comprises InGaN separate confinement heterostructure layer150has a typical thickness of about 0.1 μm, InGaN multiple quantum wells160, AlGaN electron blocking layer170has a typical thickness of about 20 nm, and InGaN separate confinement heterostructure layer180has a typical thickness of about 0.1 μm (seeFIG. 1); p-type AlGaN upper cladding layer190has a typical thickness of about 0.5 μm and p-type GaN layer195has a typical thickness of about 0.1 μm.

FIG. 2dshows etching typically by CAIBE or RIE to make the typical wave guide structure by etching through p-type GaN layer195and into p-type AlGaN upper cladding layer190. A second etch by CAIBE or RIE down to refractory metal layer130is performed as shown inFIG. 2eto provide a contact area for n-metal contact134. Finally, n-metal contact134and p-metal contact136are deposited and annealed.

FIG. 3shows an embodiment in accordance with the invention of GaN VCSEL laser structure300having ELOG n- and p-refractory metal masks with a lower DBR ELOG mask. Substrate305is typically Al2O3or SiC with GaN buffer layer310having a typical thickness of about 30 nm and separating substrate305from n-GaN layer315with a typical thickness of about 1 μm to about 2 μm. Lower dielectric distributed Bragg reflector (DBR)318overlies GaN buffer layer310. ELOG n-GaN layer320with a typical thickness of about 3 μm overlies lower dielectric DBR318and n-refractory metal ELOG mask330is a layer that overlies ELOG n-GaN layer320. N-refractory metal ELOG mask330may be made from Ti, Pt, W, Re Mo, Cr, Ni, Pd or other suitable refractory metal. Care must be taken to place n-refractory metal ELOG mask330at a null of the standing wave set up between lower dielectric DBR318and upper dielectric DBR319. N-refractory metal ELOG mask330typically has a thickness of about 50 nm or less. ELOG layer n-GaN340with a typical thickness of about 1 μm to about 2 μm, InGaN multiple quantum well active region345, p-AlGaN layer346having a typical thickness of about 20 nm and p-GaN layer350with a typical thickness of about 1 μm to about 2 μm overlie refractory metal ELOG mask330. P-refractory metal ELOG mask321is a layer that overlies p-GaN layer350. Care must be taken to place p-refractory metal ELOG mask321at a null of the standing wave set up between lower dielectric DBR318and upper dielectric DBR319. P-refractory metal ELOG mask321typically has a thickness of about 50 nm or less. P-doped refractory metal ELOG mask321may be made from Ti, Pt, W, Re Mo, Cr, Ni, Pd or other suitable refractory metal. P-GaN layer360with a typical thickness of about 1 μm to about 5 μm overlies p-doped refractory metal ELOG mask321and upper dielectric DBR319. N-metal contact334contacts n-refractory metal layer330and p-metal contact336contacts p-refractory metal layer321to provide efficient current injection into VCSEL structure300.

Typically, GaN VCSEL structure300is made by taking substrate305, typically Al2O3or SiC and depositing GaN buffer layer310to a typical thickness of about 30 nm over it. Then planar n-type GaN layer315is deposited to a thickness of about 1 μm to about 2 μm over GaN buffer layer310as shown inFIG. 4a. Lower dielectric DBR318is then deposited and patterned as shown inFIG. 4b. Lower dielectric DBR318serves as an ELOG mask for ELOG of n-GaN layer320having a typical thickness of about 3 μm and is shown inFIG. 4c. Then n-refractory metal ELOG mask330is deposited and patterned as shown inFIG. 4d. With reference toFIG. 4e, n-refractory metal ELOG mask330is then used to ELOG grow n-type GaN layer340having a typical thickness of about 1 μm to about 2 μm, InGaN multiple quantum well active region345, p-type AlGaN layer346with a typical thickness of about 20 nm and p-type GaN layer350with a typical thickness of about 1 μm to about 2 μm. After growing p-doped GaN layer350, p-refractory metal ELOG mask321is deposited on p-doped GaN layer350and patterned as shown inFIG. 4f. ELOG of p-doped GaN layer360is then performed to a typical thickness of about 1 μm to about 5 μm using p-refractory metal ELOG mask321as shown inFIG. 4g. Upper DBR319is then deposited on p-doped GaN layer360and etched as shown inFIG. 4h. Finally, as shown inFIG. 4i, etches are performed down to refractory metal layers321and330where n-electrode334and p-electrode335are deposited, respectively.

FIG. 5shows an embodiment in accordance with the invention of GaN VCSEL laser structure500having an ELOG p-refractory metal mask and using a lower DBR deposited on an n-GaN layer after removal of the substrate by laser liftoff or other suitable technique. N-type GaN layer520with a typical thickness of about 4 μm has n-contacts534attached on the bottom surface along with lower DBR518. InGaN multiple quantum well active region545overlies n-type GaN layer and is topped by AlGaN layer546having a typical thickness of about 20 nm. P-type GaN layer547having a typical thickness of about 0.2 μm to about 2 μm overlies AlGaN layer546. P-refractory metal ELOG mask535is a layer that overlies p-type GaN layer547and ELOG p-type GaN layer560having a typical thickness of about 1 μm to about 4 μm overlies p-type GaN layer547. Care must be taken to place p-refractory metal ELOG mask535at a null of the standing wave set up between lower dielectric DBR518and upper dielectric DBR519. P-refractory metal ELOG mask321typically has a thickness of about 50 nm or less. Upper DBR mirror519sits on ELOG p-type GaN layer560and p-type electrodes536are attached to p-type refractory metal layer535.

Typically, GaN VCSEL structure500may be made by taking substrate505, typically Al2O3or SiC and depositing GaN buffer layer510to a typical thickness of about 30 nm over it. Planar growth is performed for n-type GaN layer520having a typical thickness of about 4 μm, InGaN multiple quantum well active region545, AlGaN layer546having a typical thickness of about 20 nm and p-type GaN layer547with a typical thickness of about 0.2 μm to 2 μm as shown inFIG. 6a. P-refractory metal ELOG mask535is deposited on p-type GaN layer547and patterned as shown inFIG. 6b. Then ELOG growth of p-GaN layer560to a typical thickness of about 1 μm to about 4 μm is performed as shown inFIG. 6c. Upper DBR519is deposited on p-GaN layer560and patterned as shown inFIG. 6d. Substrate505is subsequentally removed by laser liftoff leaving the VCSEL structure shown inFIG. 6e. Lower DBR518is deposited on the bottom of GaN buffer layer510and patterned as shown inFIG. 6f. Finally, an RIE or CAIBE etch is performed through p-GaN layer560down to p-refractory ELOG metal mask535to deposit p-type electrodes536and n-type electrodes534on the bottom of GaN buffer layer510.